Wireless communications systems are designed around various modulation schemes, such as orthogonal frequency-division multiplexing (OFDM) and code division multiple access (CDMA), intended to provide efficient utilization of the allocated spectrum. Spectrally efficient modulation schemes have high crest factors (e.g., peak to average power ratios). However, proper conveyance of data and acceptable spectral re-growth characteristics place a linearity burden on the transmit chain, including a power amplifier.
In order to achieve the required linearity, conventional systems typically require substantial power back-off from saturation of an output transistor in the power amplifier, which significantly reduces efficiency. In portable equipment, such as cellular telephones, reduction in efficiency translates into shorter battery life and reduced operating time between battery recharges. Generally, the industry trend is to increase the interval between battery recharges and/or to decrease the size of the batteries. Therefore, the efficiency of power amplifiers should be increased while still meeting linearity requirements.
Attempts have been made to improve linearity focusing on providing two levels of drain (or collector) supply voltages. For example, U.S. Pat. No. 8,174,313 to Vice, issued May 8, 2012, which is hereby incorporated by reference, discloses controlling a power amplifier using a detector, configured to detect the power level of a radio frequency (RF) input signal with respect to a predetermined power threshold and a controller configured to provide a supply voltage to the power amplifier in response to a detection signal from the detector. The supply voltage has either a low voltage value when the detection signal indicates that the power level is below the power threshold, or a high (boosted) voltage value when the detection signal indicates that the power level is above the power threshold. However, greater efficiency and simpler design are desirable.